When you load the pants into the scene, do they fit the figure in zero pose perfectly?  In what frame did you apply the pose?  You need to give poser at least 20 frames to work with.  So your pose should be applyed at the 20th or so frame.  Here is a tutorial I wrote.  Scroll down to the "Running Future Simulations" section and follow the steps.  Your figure went into the final pose too early.  I think that's the problem.

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